PTEN is a tumor suppressor that negatively regulates the phosphoinositide 3-kinase (PI3K) signaling pathway, a central mediator of signal transduction for growth, proliferation and cell survival. PTEN is inactivated by somatic mutation in diverse human tumors including glioblastoma, endometrial carcinoma, and prostate carcinoma. Germline mutations in PTEN result in a number of phenotypic abnormalities with variable penetrance including macrocephaly, hamartomas in multiple tissues, cancer predisposition, and neurological abnormalities. Thus PTEN inactivation has consequences in multiple organ systems, and causes tumorigenesis and developmental abnormalities in the nervous system. We plan to determine Pten function in the regulation of normal and neoplastic growth in the brain. Towards this goal, we used cre-lox technology to selectively inactivate Pten in granule neurons of the cerebellum and dentate gyrus in mouse. Our preliminary data showed that Pten deficiency results in a dramatic loss of neuronal size regulation and abnormalities in cell migration. Unexpectedly, we did not observe differences in neuronal proliferation and survival despite the reported role of the PI3K pathway in these processes in granule cells. We hypothesize that Pten is critical for the appropriate control of downstream effectors required for both cell growth control and tumor suppression. We propose studies to determine the contribution of the downstream effectors Akt, mTor and S6k to the Pten-mediated regulation of neuronal size. We will also identify other gone targets that are involved in growth regulation. Finally, we will determine the effects of Pten deficiency on cell growth, proliferation, survival, and tumorigenic potential in astrocytes, the cell background that gives rise to PTEN-deficient glioblastomas. We will determine if the same signaling pathways that contribute to aberrant regulation of cell size in post-mitotic neurons are critical to Pten function in normal and neoplastic growth in astrocytes. Novel animal models with selective and inducible expression of cre recombinase will also be developed to allow analysis of Pten function at different stages of development. Our results will be integrated with results from the other projects in this program to determine which aspects of growth regulation in granule cells contribute to tumorigenesis in this cell type to give rise to medulloblastoma.